With increasing traffic in a communication circuit, the signal transmission speed of a trunk line optical transmission system has become higher year by year, and an increasing number of requests to implement a 100 Gbps next-generation optical transmission system have been issued lately.
When a signal transmission speed becomes higher, there occurs the problem that the degradation of signal quality develops by each of the factors of:    1) the reduction of the tolerancce relating to an optical signal to noise ratio;    2) an insufficient band of the wavelength filter during the WDM (wavelength division multiplexing) transmission;    3) the wavelength dispersion of a transmission line, a waveform distortion by a nonlinear effect, etc.
One of the means for solving the problems is a digital optical coherent transmission system which has recently attracted attention as an improvement of the tolerance against the waveform distortion of an OSNR and a transmission line (D. Ly-Gagnon, IEEEE JLT, pp. 12-21, 2006).
FIG. 1 is an example of a configuration of a digital optical coherent receiver.
In FIG. 1, the digital optical coherent receiver detects signal light and local light from a local light source 10 at 90° hybrid circuits 12-1 and 12-2 after splitting the light into each polarizing axis through polarization beam splitters 11-1 and 11-2. O/E converters (optical/electrical converters) 13-1 through 13-4 are provided to convert an optical signal corresponding to the amplitude and the phase of the optical signal output from the 90° hybrid circuits 12-1 and 12-2 into an electric signal. Additionally, A/D converters 14-1 through 14-4 for quantizing an electric signal are provided, and a DSP (digital signal processor) 15 is provided to compensate for a waveform distortion and demodulate a signal using quantized digital data. Thus, the polarization beam splitters 11-1 and 11-2, the 90° hybrid circuits 12-1 and 12-2, and the O/E converters 13-1 through 13-4 configure an O/E converter 16 (an O/E converter corresponding to 25 (FIG. 9) on the receiving side).
Unlike the system of performing direct detection by assigning the ON/OFF state of the conventional optical intensity mainly by the 10 Gbps optical transmission system, the digital optical coherent transmission system extracts the optical intensity and the phase information by the coherent transmission system. Then, by quantizing the extracted optical intensity and the phase information by the ADC, the digital signal processing circuit demodulates them. Therefore, the present invention corresponds to a multivalued modulation system such as M-ary PSK (phase shift keying), QAM (quadrature amplitude modulation), etc. and a frequency division multiplexing system such as FDM (frequency division multiplexing), OFDM (orthogonal frequency division multiplexing), etc.
One of the degradation factors of the signal quality of the digital optical coherent receiver is amplitude variance of a signal of each channel. The factor of the occurrence of the variance of the signal amplitude of each channel may be a difference of an individual component such as an electric line forming the route of each channel, a 90° hybrid circuit, an O/E, etc. When the amplitude of each signal deviates from the optimum state in the ADC input stage, there occurs an influence on the quality of the A/D converted signal in the ADC.
FIGS. 2 through 4 illustrate the relationship between the quality of an A/D converted signal and the amplitude of an ADC input signal.
In these figures, an example in the DP-QPSK (dual polarization-quadrature phase shift keying corresponding to the 4 QAM of the number of polarization multiplexing N=2).
FIGS. 2 and 3 illustrate the difference by the presence/absence of the variance of the signal amplitude between an in-phase (I) signal and a quadrature (Q) signal. Relating to the arrangement of the signal on the IQ constellation map, FIG. 2 illustrates in the part (a) the ideal state of the IQ signal, and FIGS. 3 and 4 illustrate in the respective parts (a) the state in which there occurs a variance of signal amplitude.
The outline of the A/D conversion of a signal in each state in FIGS. 2, 3, and 4 in their respective parts (a) is illustrated in FIGS. 2, 3, and 4 in their respective parts (b).
FIG. 2 illustrates in the part (a) the ideal state in which the signal amplitude is appropriately input as an ADC input amplitude between the I and the Q signals. As illustrated in the part (b) of FIG. 2, since the amplitude of the input signal is in an appropriate range with respect to the range of the resolution of the ADC, the signal information is not largely impaired even after the signal is quantized as a digital signal.
FIG. 3 illustrates the excessive stage of the signal amplitude in the part (a). When the signal amplitude is excessive, the amplitude is expressed as a rectangular shape on the constellation map. That is, the I and Q signals exceed the upper limit of the output of the ADC, and are limited. Therefore, the signal is cut at a value in the I and Q directions. As illustrated the part (b) of FIG. 3, since the signal information in the area in which the input signal exceeds the range of the resolution of the ADC is lost during the quantization, there occur the sensitivity degradation of the ADC and the erroneous determination in the digital signal processing as a result.
FIG. 4 illustrates in the part (a) the state in which the signal amplitude is excessively small. As illustrated the part (b) of FIG. 4, since the effective area of the resolution of the ADC becomes smaller, the signal information is lost during the quantization, and there occur the sensitivity degradation of the ADC and the erroneous determination in the digital signal processing as a result as in the case of the excessive amplitude.
In addition, another factor of the degradation of the signal quality is skew (delay time difference) between the signals of each channel. As with the above-mentioned variance of the amplitude, the factor of the occurrence of the skew may be the individual difference of each of the components such as the electric line for the route of each channel, the 90° hybrid circuit, the O/E, the ADC, etc. up to the input stage of the DSP (digital signal processor). When there is the skew between the signals, there occurs an influence on the quality of the signal regenerated in the DSP.
FIGS. 5 and 6 illustrate the relationship between the regeneration quality of a signal and the skew. In this example, the DP-QPSK system is exemplified.
FIGS. 5 and 6 illustrate the difference by the presence/absence of the skew between the I and Q signals. The parts (a) of FIGS. 5 and 6 exemplify the timing relationship between each signal and the ADC sampling. The parts (b) of FIGS. 5 and 6 exemplify the arrangement of the signals on the IQ constellation map.
FIG. 5 illustrates an ideal state indicating no skew between the I and Q signals. As illustrated in the part (a) of FIG. 5, each of the I and Q signals is sampled with the same phase timing in the ADC, and the signal point is arranged at four points by the combination of the phase state as illustrated in the part (b) of FIG. 5.
FIG. 6 illustrates the case in which there occurs the skew between the I and Q signals. As illustrated in the part (a) of FIG. 6, when the Q signal is behind the I signal by δ, it is sampled with the timing deviating from the point at which the sampling is to be performed (white point in the part (a) of FIG. 6). In this case, the signal is arranged at a point different from the original position as illustrated in the part (b) of FIG. 6.